A so-called dye color development type heat sensitive recording sheet is well-known in the art, according to which a coupler consisting of electron donative, color assuming com of a solid acid selected from clays such as activated clay, phenol compounds, aromatic carboxylic acids, aromatic polyvalent metal salts, and the like, are brought into contact with each other by heating to obtain a developed color image by the application of the color reaction therebetween.
Generally, the heat sensitive recording sheet is required as conditions for performance thereof, which the sheet should possess, to be colorless or light colored itself, to have a fast developed color image as well as an excellent performance for color development immediately after the preparation of the sheet or after long-term storage of the sheet without the lowering thereof, to be sufficiently stable to light or moisture and further, to be prepared economically. The developer for heat sensitive recording, which has already been proposed and sheets coated with the developer have both merits and demerits from the standpoint of performance and these sheets have such drawbacks that color develops prior to heating on reproduction to produce blushing because two reactants are brought into contact with each other to be coated on a substrate, that they have poor storage stability of a developed image such as light resistance and water resistance and that color does not develop instantly on heating, which demands further an improved heat sensitive recording sheet. Particularly in recent years, an appearance of a novel developer, which has excellent rise in the instantly developed color density on heating and has gradational characteristics for use in a high-speed printer with a short heating pulse length, has been highly demanded.